The present invention relates to the field of fluid filtration, and in particular to depth filters. Specifically, the present invention relates to depth filters formed from non-woven melt-blown polymeric fibers.
Non-woven melt-blown depth filters are well known and are widely used in fluid filtration applications. Such filters can be formed by extruding softened polymeric materials through an orifice of a nozzle in a stream. Jets of gas (usually air) attenuate the polymer stream to form the fibers, which are directed toward and collected by a rotating mandrel. Fibers continue to build up on the rotating mandrel until a tubular mass of fibers of the desired size and morphology is achieved.
Depth filters of the type described may include a core member to support the fiber mass. Depth filters of this type are typically produced by placing a tubular core member over the mandrel and applying the polymer fibers directly on the core member. This process, however, is discontinuous and requires that a core member be replaced on the mandrel after each depth filter is formed.
A continuous process for producing a coreless depth filter is also known. According to this process, a coarse core layer of relatively large diameter polymer fibers are initially applied directly onto a spinning mandrel to form an inner cylindrical fiber mass. Finer polymer fibers are applied over the inner cylindrical mass to form the depth filter element. The fiber mass forming the depth filter element is continuously advanced along and off of the mandrel by a press roller located adjacent to the mandrel. The inner fiber mass of such a coreless depth filter provides sufficient structural integrity to support an outer mass of relatively fine polymer fibers and withstand the fluid pressures to which the depth filter is subjected. The outer fiber mass of the finer polymer fibers, on the other hand, comprises the filtration zone of the depth filter.
Under some circumstances, it is desirable that depth filters be capable of filtering very fine particles (e.g., 1 micron) while allowing fluid under pressure to flow through the filter with a minimum drop in pressure. Depth filters with an initial pressure drop in the range of 3-4 p.s.i.d. per gallon per minute of liquid flow per ten inch element are known.
There continues to be a need in the art for a method of continuously producing a depth filter element for use with a core member which yields a suitable depth filter capable of filtering particles 1 micron or less with a minimum pressure drop.
The present invention is directed to a composite polymer filament mass especially suitable for use in constructing a cylindrical depth filter element. In the preferred embodiment, the composite polymer filament mass is comprised of a first cylindrical mass of very small diameter polymer filaments, i.e., diameters of less than about 1.5 microns. An inner cylindrical portion of the first cylindrical mass defines a smooth inner cylindrical surface of the composite filament mass. The first cylindrical mass of filaments is surrounded by a second cylindrical mass of polymer filaments having diameters greater than 1.5 microns.